Shoo means, "go away, I hate you, you don't belong here, why are you still here, you're not supposed to be here, go away!". so instead of saying all that, you just say shoo!
The correct spelling is shoo-in, usually with a hyphen. It has been known in that spelling and with the meaning of a certain winner from the 1930s. It came from horse racing, where a shoo-in was the winner of a rigged race.
In turn that seems to have come from the verb shoo, meaning to drive a person or an animal in a given direction by making noises or gestures
These day's the term is associated with short odd's favourites in any sport
